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The shifting in my M46 manual transmission was acting up, a combination of clutch and gearbox. Idecided to change out the gearbox oil first. It had been over five years and almost 14 thousand milessince I had checked that oil level. Well, previously, I had rounded the upper brass gearbox fill plug, but Icontinued to reuse it. Big mistake 1!

I tried to remove the brass fill plug using a 12-point 13-mm box end wrench. I could not loosen the plugbecause the head had become rounded and the wrench was slipping. “No problem,” I thought. “I’ll justuse the Craftsman vise-grip tool I have.” Big Mistake 2! What was normally a 45-minute job became a23-day task.

How to Avoid the Brass Gearbox Fill Plug from Hell Fiasco(Or the best way to change the gearbox oil)

© 2009 by Frederick Su. A bytewrite LLC publication.

clutchfork

gearbox13-mm fill plug

13- mm drain plug

Figure 1. The fill and drainbrass plugs of the gearbox,M46 transmission, lookingtowards front of engine.

Figure 2. Note the round-ing of the 13-mm brass fillplug from using a 12-pointbox end wrench. This wasbefore attacking it withthe vise-grips.

Figure 3. The brass fillplug after unsuccessfulattempts to loosen itwith the vise-grips. Thesteel of the vise-gripswould cut through thebrass and slip off thehead. It also wore awaythe head.

Based on my observations and tribulations, I have some recommendations, in deteriorating order, forfolks replacing the gearbox oil in the M46 (or M47) manual transmission. (More advice: If you don’thave the proper tool, go out and buy it rather than jumping steps. Sure, it’s a headache to stop whatyou’re doing to make a special trip to the tool store. But, believe me, it’s less of a headache than esca-lating the deterioration.) These steps show a progression to the worse-case scenario.

1) Loosen the fill plug first. Why? This will ensure that you have an easy way to get new oil into thegearbox. If you can’t loosen the fill plug, you still have the old oil in the gearbox and you can still useyour car. If you did it the way I did, you would have drained all the old oil out first. But, since I couldn’tbudge the fill plug, I had no easy way to refill the gearbox and I could not use my car. (Do not startengine with no oil in gearbox, as the input shaft of the transmission spins!)

Caution: Put your car up on ramps, chock wheels, put transmission in first, set handbrake.

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2) (a) Use a 6-point 13-mm box end wrench (Figure 4) to loosen the fill plug. The6-point grabs the flats of the hex head, offering a more secure grip on the plughead than a 12-point wrench.(b) If you had already used a 12-point box end wrench and had already roundedoff the brass plug head (Figure 2), then file down the protrusions from the round-ed corners so as to get the hex sides as flat as possible. Then, use the 6-point boxend wrench.

If this fails, then . . .

3) Use something like a Craftsman Bolt-Out (Figure 5). This socket grabs the bolthead and bites deeper into the head while you attempt to loosen it.

If this fails, then . . .

Figure 4. 6-pointbox end wrench.

Figure 5. TheCraftsmanBolt-Out.

Figure 7. Traditional vise-grips with straight jaws.

Figure 6. Hex gripper Vise-Grips.

5) Try a traditional straight-jawed vise grips (Figure 7) if all of theabove have failed. You’ve got nothing to lose at this point. (I’mpretty sure, though, that you will be cutting brass from the headwhile the tool slips.)

If this fails, then . . .

4) Try using the hex-gripper Vise-Grips (Figure 6). It grips hexheads better than traditional vise-grips. (This tool actually savedmy bacon when I rounded off a brass plug on another job.)

If this fails, then . . .

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7) Use J-B Weld (Figure 8a) to fill a sacrificial 13-mm or larger box end to the problem plug head(Figure 8b). Score the plug head and wrench with a Dremel tool cutting disk (Figures 9a, b) beforeapplying the J-B Weld.

Figure 8a. J-B Weld, a steelepoxy. Follow directions onlabel. I found it to be runnywhen first mixed. You can waita few minutes before applying.Clean surfaces with acetoneor paint thinner.

Figure 8b. A sacrificial 13-mm boxend wrench was “welded” to the, by now,extremely rounded brass plug head. Unfortu-nately, I had not masked the threads andended up “welding” the threads of the plugto the threads of the gearbox. What was Ithinking? Also, after the epoxy cured, therewas a depression in the box end. So, I mixedmore J-B Weld, filled the depression, andonce again, waited for it to cure.

6) Either have the car towed to a professional mechanic who can weld something to the plug head fora better grip so as to loosen, or

Figure 9a. The scored brass fill plug. Figure 9b. The scored back side ofthe sacrificial 13-mm box end.

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8) Be sure to tape off the boundary of the plug threads with that of the gearbox (Figure 10). Supportthe wrench as in Figure 11. In the end, the J-B welded wrench seemed to be able to offer the torque toloosen the rounded plug.

As I said, I did not tape the thread boundary, thus inadvertently J-B welding the threads together.Thinking I could salvage the project, I spent many, many hours scribing and scraping the weld steelaway with various wood carving knives. In the end, I couldn’t tell how much weld steel was betweenthe threads, and I didn’t want to strip the threads out of the gearbox. So I left well enough alone.After piling one bad decision on top of another in this summer of auto discontent, leaving wellenough alone was a decision I could live with.

If you did tape the threads, once the J-B Weld has cured, make sure that the thread boundary is clean.You may have to peel and cut away as much tape as possible and use a mirror to see the whole circum-ference. You may also wish to use a wood carving knife or small pen knife to score and scrape at theboundary for any weld steel that may have accumulated there. Once you’re sure the boundary is cleanand clear, then try torquing the wrench to see if the plug breaks free. Hope it works!

Caution! If you did get J-B Weld on the threads, then I highly recommend that you go to Step 9 ratherthan risk ripping out the gearbox threads, thereby killing your transmission and putting you in aworld of hurt---timewise and moneywise.

If this fails, then . . .

Figure 10. Tape offthe thread boundary ofthe brass plug with thegearbox. Discerningreaders will notice thatthis is not the fill plugof Figure 9a. Remember,I had failed to tape thatboundary. This is a photoof the drain plug and isused for illustration.

wedge

support

Figure 11. While the J-B Weldis setting up, you need to supportthe wrench. I jammed a cardboardstrawberry box as a wedge againstthe undercarriage and wrench atthe top end; for the bottom end, Ibuilt up a support using a box andblocks of wood.

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Figure 12. The overdrive switch plugand connector atop gearbox, passen-ger side. Easily felt, hard to see. Usea pick to pry off the connector. Amirror helps.

9) Fill the gearbox by going through the overdrive switch plug (Figure 12) on top of the gearbox, pas-senger side. An IPD Tech gave me this hint. He said I would have to drop the transmission a bit to getto the 22-mm plug because space is so confined in there. But, I got around that by using a 22-mmcrowsfoot wrench from ZD-Mak (Figure 13). The wrench combination I used is shown in Figure 14.

Figure 13. The 22-mm ZD Mak tool, whichalso fits the oxygen sensor plug.

Figure 14. This is the wrench combinationI used: 3/8" socket wrench with 8" extensionand a 3/8" universal joint.

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Figure 15. A 7-mm hose is inserted into the over-drive switch hole. You need to work it around apartial obstruction.

Figure 16. The other end of the hose was con-nected to a squirt can filled with 10W-30 syn-thetic oil.

10. Insert a hose (7-mm works) into the overdrive switch hole (Figure 15) and work it around the par-tial obstruction. Hook up the other end to a squirt can (Figure 16). IPD recommends that for old cars,rather than filling with gear oil or ATF fluid (type F or G), that the gearbox be filled 10W-30 synthetic oil.Use 2.4 quarts. Put overdrive plug back in and tighten (not too tight). Hook up electrical connector.Now, you can start your engine.

11. And finally, if you have that J-B Welded wrench hanging around beneath the car, it’s time to cut itshort (Figure 17). I left a short handle, so I can slip a piece of pipe over it if I ever decide to try torquingit loose---perhaps just before the car goes to clunkerville.

Figure 17. The J-B Welded wrench,cut short. It will remain as a reminderof my stupidity.